Access tunnel and attenuator for microwave ovens

ABSTRACT

This invention relates to the treatment of materials by the application of microwave electromagnetic fields, and more particularly, to the development of a microwave absorbent tunnel which permits continuous free access into a microwave oven while limiting the escape of microwave energy therefrom.

United States Patent [191 J urgensen [451 Aug. 21, 1973 ACCESS TUNNELAND ATTENUATOR FOR MICROWAVE OVENS [75] Inventor: Peter-D. Jurgensen,San Carlos,

[21] Appl. No.: 241,238

[52] U.S. Cl. 2l9/l0.55

3,478,900 ll/l969 Jeppson ..2l9/l0.55 3,643,054 2/1972 Forster..219/l0.55

Primary Examiner-J. VITruhe 4 wfixqmirlr:liy hP; le s?! Attorney-Paul D.Flehr, Robert B. Block et al.

[57] ABSTRACT This invention relates to the treatment of materials bythe application of microwave electromagnetic fields, E2 and moreparticularly, to the development of a micro. wave absorbent tunnel whichpermits continuous free 56] References Cited access into a microwaveoven while limiting the escape of microwave energy therefrom. UNITEDSTATES PATENTS 3,050,606 8/1962 Tibbs 219/1055 9 Claims, 4 DrawingFigures Coolant In Patented Aug. 21, 1973 2 Sheets-Sheet 1 Patented Aug.21, 1973 2 Sheets-Sheet 2 ACCESS TUNNEL AND ATTENUATOR FOR MICROWAVEOVENS Microwave heating as applied to many materials can be convenientlyand economically carried out with a microwave oven. For continuousprocesses, some means of access must be provided to the oven and shouldbe compatible with the use of a conveyor system for carrying thematerial through the access and oven. A physical opening of adequatesize in the oven wall presents a leakage hazard to operating personneldue to potential bialogical defects of microwave radiation. Safetyregulations are presently proposed which would require the fieldstrength of any measured leakage from an oven to be less than milliwattsper square cm.

Previous solutions to this problem have not been completelysatisfactory. For frequencies most used, leakage control using awaveguide beyond cutoff requires that the opening be too small to bepractical for many applications. Microwave absrobent linings used inlarger tunnels are difficult to couple to the microwave fields and alsorequire a substantial length to be effective and are also difficult toproperly and adequately cool.

Access tunnels using microwave traps and reflectors have been proposed,such as cavities, capacitive structures, or slot reflectors, eitherstationary or moving .with the conveyed materials; Such traps andreflectors tend to be bandwidth limited and are not adequately coupledto the possible modes of propagation. In addition, they can becomedetuned or uncoupled by the presence of the material being treated whichrenders their effectiveness somewhat uncertain.

Another form ofprior device proposes the utilization of tunnel havinginner and outer walls, the space between which is filled with a lossydielectric liquid, the inner wall being made of a dielectric materialthrough which the microwave energy passes and is dissipated in theliquid. However, the degree of mismatch between a dielectric liquid,such as water, is such that such a de- SUMMARY OF THE INVENTION ANDOBJECTS In general, it is an object of the present invention to providean access tunnel and attenuator microwave oven system which willovercome the above limitations and disadvantages.

A further object of the present invention is to provide a microwaveaccess tunnel for a microwave oven which is characterized by beingphysically short and compact which has a higher attenuation thanpreviously available in a given size together with a high-power handlingcapability.

Another object of the invention is to provide a microwave access tunnelwhich can utilize ordinary tap water as a coolant.

In general, the foregoing objects are achieved by providing a tunnelstructure having an interior conductive wall which is in communicationwith an access port to an associated microwave oven. The wall isconstructed to form a broad-band directional coupler having slotdiscontinuities which serve to concentrate and transfer microwave energythrough that wall of the access tunnel and also at least partiallyprovide an impedance match between the interior and exterior sides ofthe coupler. Adjacent the coupler are channels of lossy dielectricmaterial preferably constructed of lossy dielectric tubing having adielectric consonant intermediate that of a coolant which is circulatedthrough the tubing and free space. By providing structure of theforegoing type, electrical discontinuities in the tunnel wall arecreated which serve to transmit energy through the wall and toconcentrate it within the adjacent lossy material and coolant to obtaina maximum absorption and a high rate of attenuation per unit length ofthe tunnel.

1n the preferred form of the invention, a plurality of spaced parallelslots positioned one-fourth wavelength apart and at an angle ofapproximately 45 to the direc tion of theoretical propagation of energyin the tunnel is found satisfactory, together with a traverse coilspacing having elements spaced approximately one-fourth wavelengthapart.

These and other objects of the invention will become apparent from thefollowing detailed description thereof when taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view partlybroken away illustrating a microwave access tunnel and oven constructedin accordance with thepresent invention.

FIG. 2 is a cross-sectional view of the microwave tunnel construction ofFIG. 1 taken along the lines 2-2 thereof. i

FIG. 3 is a cross-sectional view taken along the lines 33 of FIG. 2. 1

FIG. 4 is a cross-sectional view taken along the lines 4--4 of FIG. 2. I

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS.1 through 4, there is shown one preferred form construction implying thepresent invention and consists generally of a microwave oven having alarge access port in one wall thereof to which a microwave access tunnel14 is coupled and supported in close fitting and fixed positiontherewith by suitable means (not shown). The tunnel provides access fora conveyor belt 16 which passes through the tunnel and slides over aninterior lower wall thereof.

The microwave access tunnel l4 of the present invention is constructedof a plurality of conductive walls which may, for example, be made of asuitable sheet metal and comprise outer, top, bottom, and side walls 18,20, 22, and 24; end face walls 26; and inner top, bottom, and sideconductive walls 28, 30, 32, and 34. As shown particularly in FIGS. 1and 2, the outer top and bottom walls 18, 20 are formed by a singlesheets of metal having bent ends 36, to which are fastened the upper andlower inner conductive walls by screws 38. The outer side walls 22, 24are formed as a cap-like enclosure and are secured to extensions 40 ofthe outer top and bottom walls by screws 42, shown in FIG. 2.

The interior side walls 32, 34 are formed by suitable sheet metal stripshaving top and bottom portions 44 fastened to the top and bottom outerwalls 18, 20 by screws 46. The foregoing construction has been foundsatisfactory for providing a rigid self-contained unit which may bedetachably removable from the associated microwave oven should it bedesired to convert the same to batch operation by closing off the accessports 12.

Taken together, the various walls of the tunnel form an interiorwaveguide structure having a uniform rectangular cross section with itsaxis of propagation in line with the normal surface vector defining theaccess port. This waveguide structure is surrounded by an annularchamber which is completely closed within a conductive box or shield.

Means is provided for forming a broad band directional coupler ormatching plate for transferring microwave energy from the interior ofthe tunnel to the annular region between the exterior and interiorwalls. The coupling means is distributed throughout one broad wallinterior of the tunnel, as for example, bottom wall 30 and consists of aplurality of slots 50a 50k arranged at an angle of about 45 to thedirection of propagation of material and energy in said tunnel (itsaxis). It has been found that the slots 50 are particularly effective asa means for concentrating energy in their vicinity and therebypermitting the same to be coupled through the slots when the inter slotspacing is approximately one-quarter wavelength apart from each other asmeasured by the closest distance oriented at about 45 to the propagationdimension of the tunnel. The slotted plate 30 may be arranged in eitherone of the top or bottom walls 28, 30 or both, it having been found thatsubstantially all the requisite attenuation available from the presentconstruction is obtained by providing slots solely of walls 28, 30, asfor example, the lower wall 30 as shown in FIG. 1. It should berealized, however, that it may be advantageous to provide solely theupper wall with slots so as to avoid debris falling through the slotsunder gravity during operation of the device.

While a detailed theoretical analysis is not available, it is believedthat the coupler can be characterized as serving the function ofmatching plate which reorganizes the field directions of the otherwiserandom microwave fields entering the tunnel through port 12 so that asthey travel down the tunnel, they are realigned in a direction so thattheir continued encounter with the slot structure of the plate 30excites the slots and causes significant transference of energy into theannular chamber surrounding the interior walls of the tunnel.

In practice the slot orientation and spacing has proven exceedinglyeffective in obtaining a coupling to all propagating modes and it is notunduly frequency sensitive.

Means is provided for absorbing microwave energy transmitted through thecoupling plate 30 and includes suitable microwave absorber positioned inthe annular region outside the interior conductive tunnel walls toreceive microwave energy from a coupler. In order to satisfactorily meetrelatively high power handling demands, it is preferred that themicrowave absorber take the form of a plurality of tubes 52 positionedtransversely of the axis of transmission of the tunnel forming channelsand designed to carry a dielectric coolant 53 such as water. Each tubeis preferably constructed in part of a high loss, dielectric materialhaving a dielectric consonant intermediate that of the coolant liquidcarried through the tube and that of free space so as to moreeffectively handle microwave energy. While the tubes could beconstructed and arranged to connect to a suitable manifold structure ateach side of the tunnel, it is found to be more convenient to constructthem in a connected array resembling a coil having an inlet 54 and adischarge 56 so that fluid coolant passing through them enters throughthe inlet and passes sequentially through each turn of the coil, finallyto be discharged at the opposite end of the device. Since the greatestpower handling capability is demanded at the end of the device which isnearest to the microwave oven, it is preferred that that endbe'connected to the input or source of coolant fluid.

Referring now particularly to FIGS. 2 through 4 wherein the constructionof each tube 52 is shown in detail, each channel is formed of aninterior tubing section 58 of high loss material. An example of asuitable material is silicon rubber, heavily loaded with dispersedparticulate carbon black. In view of the strength limitations of suchmaterial, it is found desirable to surround the same with a microwavetransparent tube 60, which may, for example, be made ofpolytetrafluoroethylene and which will support the carbon-loaded siliconrubber tube against undue deformation upon application of internal fluidpressure. The end of each tube 58 is sealed into contact with a U-shapedturnaround 62 which consists of silicon rubber tubing which extends intotube 52 a length sufficient to be within the outer support tube and issealed to the former with a suitable adhesive sealant. Supportpositioning of the assembled is obtained by providing a plurality ofholes 64 appropriately spaced and aligned in the upper and lowerportions of the interior side walls 32, 34, such portions being of lowel ecyrical consequence and provided solely for general support of theunit of the tubes 52, 60. As shown, each turnaround shifts one-half atubing width so that each complete turn of the coil advances or shiftsthe coolant from one adjacent tube to the next. The size of the tubingand the inter spacing between holes 64 is such that the spacing betweenturns of adjacent channels is approximately one-quarter wavelength andsuch that essentially all available space to locate absorber isutilized.

After an assembly and provision for suitable structional support of theoperation of the access tunnel of the present invention requires onlythe existence of a suitable coolant fluid to the input line. In oneapplication which was designed for use at 2,450 MHz at a microwave powerrating up to 5,000 watts required 2 gallons per minute of tap water at apressure of 45 pounds per square inch, well within usual municipalsupply line capacity, it was found that a device constructed inaccordance with the present invention permitted no load operationcontinuously in the: oven itself at full power in a conveyor mode. Themicrowave leakage was less than l0 milliwatts per square cm at 2 inchesfrom any part of the system. The overall length of the microwave tunnelcapable of the foregoing specifications was 18 inches.

While there has been disclosed the use of a slotted conductive plate asa directional coupler, it will be realized that resonant slots, ororifices of many kinds may be capable of serving the purposes requiredin the present invention. These requirements include impedance matchingfrom the interior of the tunnel through a wall,

ability to concentrate field energy with the adjacent absorber bydirecting a component of the total energy flow normal to the plate andthrough its wall. Also, while certain absorber materials have beendisclosed as preferred from those now available, they may be replaced byothers, particularly as higher strength materials become available.

Accordingly, the scope of the invention should not be limited to thespecific structure shown as the preferred embodiment herein by way ofexample, but should be interpreted with reference to the accompanyingclaims.

I claim:

1. In a microwave system for the treatment of materials by theapplication of microwave electromagnetic fields, a microwave oven, meansforming at least one access port to said oven including, an accesstunnel and attenuator for use therewith comprising means forming aplurality of conductive walls constructed and arranged to couple to saidport and to provide an access tunnel thereto, means for forming adirectional coupler for transferring microwave energy from the interiorof said tunnel to the exterior thereof, said coupler means beingdistributed throughout at least said one wall of said tunnel, amicrowave absorber positioned outside of said conductive tunnel walls toreceive microwave energy from said coupler.

2. A system as in claim 1 in which said microwave absorber includesmeans forming a plurality of channels, tubes for carrying a dielectriccoolant therethrough, portions of said channels being formed of a lossydielectric material having a dielectric constant intermediate that ofsaid lossy liquid and free space, said lossy dielectric materialportions of said coil being arranged immediately adjacent that side ofsaid directional coupler on the outside of said conductive walls.

3. A system as in claim 2, in which said channels are spaced one-quarterwavelength apart.

4. A system as in claim 2, in which the portion of said channel adjacentsaid directional coupler includes a tubing made of dielectric materialhaving high loss at microwave frequencies.

5. A system as in claim 4, wherein said lossy material is a siliconrubber loaded with carbon particles.

6. A system as in claim 5, wherein said channel further includes atubular section made of material transparent to microwaves andsurrounding said lossy tubing to support the same.

7. In a microwave system for the treatment of materials by theapplication of microwave electromagnetic fields, a microwave oven, meansforming access port to said oven including, an access tunnel andattenuator for use therewith comprising means forming a plurality ofconductive walls constructed and arranged to couple to said port and toprovide an access tunnel thereto, means associated with one of the wallsof said tunnel for forming a multi mode directional coupler fortransferring microwave energy from the interior of said tunnel to theexterior thereof, said coupler means being formed by a plurality ofslots formed in at least one wall of said tunnel, and arranged at anangle of about 45 to axis of said tunnel, a microwave absorberpositioned outside of said conductive tunnel walls to receive microwaveenergy from said coupler.

8. A microwave system as in claim 7, in which said slots are spacedapart approximately one-quarter wavelength.

- 9. In a microwave system for the treatment of materials by applicationof microwave electromagnetic field, a microwave oven, means formingaccess port to said oven, and access tunnel and attenuator for usetherewith comprising means forming a plurality of conductive wallsconstructed and arranged to couple to said said coupler.

1. In a microwave system for the treatment of materials by theapplication of microwave electromagnetic fields, a microwave oven, meansforming at least one access port to said oven including, an accesstunnel and attenuator for use therewith comprising means forming aplurality of conductive walls constructed and arranged to couple to saidport and to provide an access tunnel thereto, means for forming adirectional coupler for transferring microwave energy from the interiorof said tunnel to the exterior thereof, said coupler means beingdistributed throughout at least said one wall of said tunnel, amicrowave absorber positioned outside of said conductive tunnel walls toreceive microwave energy from said coupler.
 2. A system as in claim 1 inwhich said microwave absorber includes means forming a plurality ofchannels, tubes for carrying a dielectric coolant therethrough, portionsof said channels being formed of a lossy dielectric material having adielectric constant intermediate that of said lossy liquid and freespace, said lossy dielectric material portions of said coil beingarranged immediately Adjacent that side of said directional coupler onthe outside of said conductive walls.
 3. A system as in claim 2, inwhich said channels are spaced one-quarter wavelength apart.
 4. A systemas in claim 2, in which the portion of said channel adjacent saiddirectional coupler includes a tubing made of dielectric material havinghigh loss at microwave frequencies.
 5. A system as in claim 4, whereinsaid lossy material is a silicon rubber loaded with carbon particles. 6.A system as in claim 5, wherein said channel further includes a tubularsection made of material transparent to microwaves and surrounding saidlossy tubing to support the same.
 7. In a microwave system for thetreatment of materials by the application of microwave electromagneticfields, a microwave oven, means forming access port to said ovenincluding, an access tunnel and attenuator for use therewith comprisingmeans forming a plurality of conductive walls constructed and arrangedto couple to said port and to provide an access tunnel thereto, meansassociated with one of the walls of said tunnel for forming a multi modedirectional coupler for transferring microwave energy from the interiorof said tunnel to the exterior thereof, said coupler means being formedby a plurality of slots formed in at least one wall of said tunnel, andarranged at an angle of about 45* to axis of said tunnel, a microwaveabsorber positioned outside of said conductive tunnel walls to receivemicrowave energy from said coupler.
 8. A microwave system as in claim 7,in which said slots are spaced apart approximately one-quarterwavelength.
 9. In a microwave system for the treatment of materials byapplication of microwave electromagnetic field, a microwave oven, meansforming access port to said oven, and access tunnel and attenuator foruse therewith comprising means forming a plurality of conductive wallsconstructed and arranged to couple to said port and to provide an accesstunnel thereto, a conveyor disposed through said tunnel and oven forpassing material to be treated through said oven, means associated withone of the walls of said tunnel for forming a multi mode directionalcoupler for transferring microwave energy from the interior of saidtunnel to the exterior thereof, said coupler means being locatedthroughout at least said one wall of said tunnel, microwave absorbermeans positioned outside of said conductive tunnel walls to receivemicrowave energy from said coupler.